Humans have fewer viral remnants in their genes than other mammals because they gave up biting, research suggests.
When our ancestors started using weapons to fight and hunt with instead of teeth they reduced their exposure to blood-borne viruses, British scientists believe.
As a result, humans are unusual in not having had any new types of virus embedded in their DNA in the last 30 million years.
Certain viruses known as "retroviruses" are capable of incorporating themselves into an animal's genetic code and being passed down from one generation to the next.
These "endogenous retroviruses" (ERVs) are not a serious threat to humans but can result in diseases such as cancer in other animals.
A team of researchers from Oxford University, the University of Plymouth and the University of Glasgow compared the number of times ERVs appeared to have entered the genomes of animals and 39 other mammalian species, including chimpanzees, dolphins and great pandas.
By tracking the viruses' mutation rate, the researchers were able to measure how long a virus had remained in a particular animal's genome.
They found that compared with other animals, far fewer retroviruses had been incorporated into the DNA of humans and great apes over the last 10 million years.
Humans were unique in not having acquired any new ERVs in 30 millennia.
One reason could be a change in behaviour as humans evolved, the scientists said.
Fewer close-up and personal bloody fights using teeth and claws meant less exposure to the viruses.
Dr Robert Belshaw, from the University of Plymouth, said: "Considering us simply as a primate species, the proportion of human individuals infected with retroviruses is much less than among our relatives such as chimpanzees."
The research is published in the journal Retrovirology.
